Swards Of Subterranean Clover Research Articles

Monitoring ley pastures and their response to winter cleaning

The purposes of this study were to evaluate subterranean clover-based leys on farms and in experiments using several pasture parameters, and to assess the impact of winter cleaning on the productivity and botanical composition of clover swards. Annual pastures were monitored on a group of 5 farms in the Wagga district and compared with an experimental subterranean clover (Trifolium subterraneum L.) pasture. The major problem in the farm paddocks was a lack of legume biomass due to poor legume densities, a consequence of the use of the soft-seeded cultivar Woogenellup and a high content of grassy weeds. The farmers in the group were unaware of the tools, parameters and benchmarks for making quantitative pasture comparisons. In 2 experiments, a range of subterranean clover swards were generated through the use of cultivars, seeding rate and winter cleaning treatments, grazed at 15 sheep/ha and monitored for 3 years. Appropriate benchmark values for the seed pool of subterranean clover were 300–350 kg/ha in winter and 600–700 kg/ha in summer. On the basis of both winter production, a function of May seedling density (target >1000 seedlings/m2) and spring production, which depended on the cultivar maturity, Junee was superior at Wagga to either Dalkeith (earlier maturing) or Woogenellup (softer seeded). Winter cleaning, using selective herbicides (fluazifop, simazine) to remove grasses and weeds, was advantageous in achieving a high content (>90%) and productivity of subterranean clover, provided that the legume content of the pasture was at least 28%, or >20% of total ground area before herbicide application in winter. In winter-cleaned swards, legume growth increased by up to 80%, legume biomass was improved by up to 46% and legume content increased from <50 to >95%. The main disadvantages of winter cleaning were increased areas of bare ground and reduced total biomass for several weeks after herbicide application, and the rapid development of ryegrass that was resistant to at least 1 of the herbicides used. The strategic use of observations to monitor the performance of pastures and their response to management is discussed.

The effect of continuous grazing on the seed production of Subterranean clover (Trifolium subterraneum).

Three pure swards of subterranean clover were either (a) left ungrazed with a leaf area index (LAI) of 4.6, or (b) continuously grazed leniently to leave 1 600 kg DM/ha herbage mass equivalent to an LAI of 1.0, or (c) hard grazed to leave 1 400 kg DM/ha herbage mass equivalent to an LAI of 0.6 throughout the flowering period. These swards yielded 1 250, 320 and 70 kg/ha of seed, respectively. One intensive sampling during the middle of flowering showed that seed yield was positively correlated with plant weight, leaf area per plant, and area per leaf, specific leaf area and leaf area ratio. Grazing reduced numbers of both inflorescences and burrs and hence seed yield. Whilst reduced leaf area and plant weight may have also been partly responsible, under hard grazing, only 58% of potential reproductive sites were occupied which suggests that grazing animals also reduced seed yield by consuming these. It is recommended that continuous grazing during flowering and seed maturation should be controlled to leave at least 1 600 kg DM/ha of residual herbage mass, equivalent to an LAI of 1.0, if adequate seed is to be produced to be ensure the establishment of a high producing sward in the following growing season. Key words: flowering, grazing, herbage mass, leaf area, seed production, Trifolium subterraneum

Productivity of Trifolium subterraneum and Phalaris aquatica under warmer, high CO2 conditions

Summary Despite the importance of grass‐legume pastoral ecosystems worldwide, there is little known about the impacts of concurrent increase in temperature and atmospheric CO2 concentration on their productivity. Pure and mixed swards of subterranean clover (Trifolium subterraneum) and phalaris (Phalaris aquatica) were established under ambient and warmed (+3.4°C) air temperatures, at ambient and 690 µmol mol−1 CO2 concentrations in field tunnels in temperate south‐eastern Australia. Over one year, elevated CO2 increased clover foliage growth in the monoculture by 19%, and by 31% in the mixture. Warming reduced clover monoculture herbage production at ambient CO2 by 28% and reduced the growth enhancement by elevated CO2 to +8%. Forage growth of phalaris monoculture was not affected significantly by either factor. Forage growth of the mixture was increased by 34% in response to higher CO2, but unaffected by warming. Elevated CO2 combined with warming increased forage growth of the mixed sward by 23%. Concurrent rise in atmospheric CO2 concentration and temperature increased productivity of subterranean clover‐phalaris swards. However, longer term effects on species competition and persistence may modify this conclusion.

Nutritive value and the nitrogen dynamics of Trifolium subterraneum and Phalaris aquatica under warmer, high CO2 conditions

Summary Will changes in nutritive values and N‐relations offset initial gains in forage productivity under potential climate change observed for grass–legume pastures of south‐eastern Australia? Herbage nutritive value and symbiotic nitrogen fixation were investigated for pure and mixed swards of subterranean clover (Trifolium subterraneum) and phalaris (Phalaris aquatica) in field tunnels at ambient and 690 µmol mol−1 CO2 concentrations and at ambient and warmed (+3.4°C) air temperatures. Elevated CO2 increased the nonstructural carbohydrate content of herbage whereas warming tended to decrease it. These effects were mainly on soluble carbohydrates in phalaris and starch in clover herbage. The N concentration of both species was decreased by elevated CO2 but unaffected by warming. The proportion of clover‐N derived from N2 fixation was increased by 12% under elevated CO2 but decreased by 6% under warming. Concurrent warming and high‐CO2 conditions are expected to lead to improved herbage nutritive value for ruminants due to increased nonstructural soluble carbohydrate content. Longer term effects on nutritive value and N‐dynamics via species persistence and competition require further study.

Effect of perennial pasture species on surface soil moisture and early growth and survival of subterranean clover (Trifolium subterraneum L.) seedlings

Subterranean clover seedling numbers and growth in swards containing 1 of 5 perennial pasture species [phalaris (Phalaris aquatica) cv. Sirolan, cocksfoot (Dactylis glomerata) cv. Currie, lucerne (Medicago sativa) cv. Aquarius, wallaby grass (Danthonia richardsonii) cv. Taranna, and lovegrass (Eragrostis curvula) cv. Consol] were compared with those in typical annual pastures and pure clover swards in the wheatbelt of eastern Australia. Presence of a perennial species or the volunteer annual grass (Eragrostis cilianensis) increased the rate of drying of the soil surface (0–5 cm) after late February and May rain, compared with subterranean clover swards. Perennials differed in the rate they dried the soil surface, with the more summer-active lucerne and consul lovegrass drying the profile more rapidly than phalaris. The amount of water in the surface 5 cm, 6 days after the rainfall event on 27–28 February, was strongly negatively correlated (r = –0·75, P < 0·01) with the amount of green perennial biomass, but not related to standing dead material or surface residues. Where perennials were present, a smaller proportion (2–4%) of the clover seed pool produced seedlings in response to late summer rain, compared with pure clover swards (18%). A higher proportion of the seed pool produced seedlings (19–36%) following rain in late autumn but there was no difference between species. The more summer-active perennials (cocksfoot, danthonia, and lucerne) markedly depressed the survival of emerged clover seedlings following both germinations. Of the seedlings that emerged in early March, the proportion remaining by 29 March was 57% in phalaris, 21% in lucerne, 13% in danthonia, and 1% in cocksfoot, compared with a 78% increase in seedlings in pure subterranean clover swards. By 15 May, all perennials had <2 clover seedlings/m2 surviving, compared with 37 in the annual pasture and 964 plants/m2 in pure subterranean clover. Following the May germination, the highest proportion of emerged seedlings surviving until 29 May was in the phalaris swards (40%) and least in the cocksfoot and danthonia swards (2–4%). Presence of a perennial or annual grass decreased (P < 0·05) relative water content of clover seedlings on 15 March from 74% in pure clover swards, to 48% in annual pasture, 34% in phalaris, and 29% in lucerne swards. Clover seedlings growing in pure subterranean swards on 15 March (17 days after germinating rain) were 4 times larger than those in lucerne and twice as large as those in either phalaris or annual pasture. Seed size did not differ between treatments, but available mineral soil nitrogen was significantly higher (P < 0·001) in pure subterranean clover swards (32 mg N/g) compared with perennials (3–13 mg N/g). Strategies such as heavy grazing in late summer to reduce green biomass of the perennials or sowing the perennials at lower densities may reduce the adverse effects that perennials have on subterranean clover seedlings in these drier environments.

Change in stomatal resistance and water use in subterranean clover (Trifolium subterraneum L.) in response to broadleaf herbicides

The effect of 5 broadleaf herbicides on the water use and stomatal resistance of 2 cultivars of subterranean clover (Trifolium subterraneum L.) was examined in a glasshouse study. The herbicide treatments 2,4-DB, MCPA, bromoxynil, MCPA+terbutryn, and MCPA+diuron were applied at 6 rates at 2 times (14 May, 14 June) to plants at 2 leaf stages (3-4 and 8-10 leaves). Each of the herbicides reduced water use by the clover within 24 h, the size of the reduction increasing with the rate of herbicide applied. The herbicide treatments MCPA+terbutryn, MCPA+diuron, and bromoxynil caused the largest reductions (44-52%) in total water use over the 30-day period when applied at the recommended rate, and MCPA and 2,4-DB the least reduction (16-22%). Stomatal resistance increased substantially within 2 days of application of each of the herbicides. The magnitude of the change differed with herbicide and increased with herbicide rate. The effect of the herbicides on stomatal resistance declined 10-20 days after herbicide application in all treatments except 2,4-DB, but stomatal resistance of all herbicide-treated plants was still higher than the control 30 days after herbicide application. The herbicides LICPA+terbutryn and LlCPA+diuron and bromoxynil caused the largest increase in stomatal resistance and 2,4-DB the least. Stomatal resistance was found to be highly negatively correlated with daily water use by the clover plants at 2 days (r = -0.84, P < 0.01) and 30 days (r = -0.88, P < 0.01) after herbicide application. All of the herbicides reduced the LA1 of the plants, the effect increasing as the herbicide rate increased. Herbicide and herbicide rate had the largest effect on both water use and stomatal resistance; the effect of cultivar, leaf stage, and spraying time accounted for a relatively small proportion of the variance. The findings support the hypothesis that some broadleaf herbicides can result in a water-saving effect in subterranean clover swards through increasing stomatal resistance and decreasing the LAI, thereby potentially reducing moisture stress during seed set.

Losses in herbage and seed yields caused by subterranean clover mottle sobemovirus in grazed subterranean clover swards

Subterranean clover (Trifolium subterraneum L.) was grown in swards into which small plants infected with subterranean clover mottle sobemovirus (SCMoV) were transplanted. Initial spread was mainly to plants adjacent to infected transplants and occurred during grazing and trampling by sheep. New sites of infection subsequently formed away from the original infection sites. Spread occurred mainly in spring and its extent varied with cultivar, site and year. Final SCMoV incidence in swards ranged from 17% of plants infected by isolate MJ in cv. Karridale in 1993 to 60% by isolate P23 in cv. Dinninup in 1991. With cvv. Dinninup and Junee, in two experiments SCMoV-infection decreased yields within patches with symptoms by 11-26% (herbage) and 5-31% (seed); in one of these experiments infection decreased overall yields of herbage by 9-10% and seed by 10-31%. In an experiment with cvv. Dalkeith and Woogenellup, SCMoV-induced overall herbage yield decreases of 10-13% were recorded despite extensive virus spread to control swards; yield losseswithin affected patches were 20-28% for herbage and 19% for seed. In an experiment with cw. Karridale and Woogenellup, SCMoV-induced yield losses within patches with symptoms were 38-44% for herbage and 15-41% for seed. Decreased seed size (mean seed weight) and fewer seeds both contributed to the diminished seed yields. Estimates of the effects of different levels of SCMoV infection on herbage yields were obtained for cv. Woogenellup by plotting dry weight data from (i) individual quadrats against percent plants with symptoms within them and (ii) whole plots against percent plot infection determined by ELISA. Losses increased in proportion to the level of infection. SCMoV infection of subterranean clover pastures not only decreases the feed available but also the seed bank which, when compounded over several years, leads to pasture deterioration. Heavy grazing, reseeding the pasture with susceptible cultivars and extended growing seasons are all likely to magnify SCMoV-induced losses.

Plant population dynamics in subterranean clover and murex medic swards. I. Size and composition of the seed bank

It is possible that the use of harder-seeded varieties of sub. clover (Trifolium subterraneum) and the recently domesticated murex medic (Medicago murex) may improve annual legume persistence in pasture leys. To determine the mechanisms of production and persistence in these two species, the size and composition of the seed bank were compared in four varieties of sub. clover and two lines of murex medic over a three-year period at Forbes in central-west N.S.W. The rate of breakdown of hardseed and the loss of high temperature dormancy was also studied in the same genotypes under conditions of alternating temperature in controlled environment cabinets. In terms of the size of the total seed bank, the sub. clover cultivar with the highest level of hardseed (Dalkeith) was the most successful genotype studied. This was the result of slower rates of hardseed breakdown compared to the other sub. clover cultivars, and better seed production compared to the two murex medic lines. Losses of seed from the seed bank were substantial, and were not all accounted for by germinated seedlings or grazing over summer. The sub. clover cultivars Junee and Seaton Park had the highest levels of high temperature embryo dormancy and the lowest proportion of seedlings germinating over summer. The level of sub. clover hardseed breakdown in the field corresponded well with the 60/15�C laboratory estimates of hardseed breakdown, even though soil surface temperatures over summer at Forbes were rarely in the range of 60/15�C.

Infection of subterranean clover swards with bean yellow mosaic potyvirus: losses in herbage and seed yields and patterns of virus spread

During 1989-92, subterranean clover ( Trifolium subterraneum L.) was grown in field experiments in which swards of six cultivars were infected with bean yellow mosaic potyvirus (BYMV) by transplanting small BYMV-infected subterranean clover plants into them. The swards were then grazed by sheep or mown to simulate grazing. The infected transplants were the primary virus source for subsequent spread by aphids. Spread initially centred on infected transplants resulting in circular expanding infected patches. Later, secondary patches, isolated affected plants and more generalized infection sometimes developed. The extent of spread within swards from the transplants varied with cultivar, BYMV isolate, site and year. Final BYMV incidence ranged from 12% of plants symptom-affected by isolate MI in cv. Junee in 1991 to 100% by isolate SMB in cv. Leura in 1992. BYMV spread mostly occurred in spring and was increased around the edges of areas of bare ground in swards. In two experiments at one site in which 'mini swards' of cvv. Green Range, Karridale and Leura were mown repeatedly, BYMV-infection decreased herbage yields (dry weights) by 12-16% while seed yields were decreased significantly (by 37-40010) in one experiment. In a grazing experiment at a second site with swards of cvv. Esperance and Karridale, BYMV-infection decreased overall yields of herbage by 18-39% and seed by 11-12%; herbage yield losses within symptom-affected patches were 28-49%. In a further grazing experiment at this site with swards of cvv. Junee and Karridale, BYMV-induced losses determined from symptom-affected patches were 21-29% for herbage and 15-25% for seed. In a grazing experiment with swards of cvv. Denmark and ~ e u r a ' at a third site, BYMV-induced overall herbage yield decreases of 8-12% were still recorded despite extensive BYMV spread to control swards; yield losses within symptom-affected patches were 18-25% for herbage and 35-47% for seed. Seed yield losses were due to decreased seed size (mean seed weight), fewer seeds being produced, or both. Estimates of the effects of different levels of BYMV infection on herbage yields in partially infected grazed swards were obtained for cvv. Denmark, Karridale and Leura by plotting individual quadrat data for herbage dry weights against % symptom-affected plants. Losses increased in proportion to the level of infection, but their magnitude also varied with cultivar and experiment. It is concluded that BYMV infection of subterranean clover pastures is cause for concern, not only as regards herbage yield losses but also as regards depletion of the seed bank, which, when compounded year by year, results in pasture deterioration. Early and prolonged aphid activity, reseeding the pasture with susceptible cultivars, heavy grazing and extended growing seasons are all likely to magnify BYMV-induced losses.

Losses in productivity of subterranean clover swards caused by sowing cucumber mosaic virus‐infected seed

SummaryField trials were done in 1988 ‐ 89 at two sites to examine the effects of sowing seed stocks in which a low proportion (1.6–7.0%) of the seed was carrying cucumber mosaic virus (CMV) infection (= infected seed) and the subsequent CM V spread that results, on the productivity of swards of subterranean clover cvs Esperance, Green Range and Karridale. Except in irrigated plots of cv. Green Range, a variable proportion of the CMV‐infected seedlings always failed to establish, so sowing infected seed normally resulted in plots containing fewer or far fewer seed‐infected plants than expected. The rate of virus spread by aphids was faster and resulted in more extensive infection at maturity when the plots contained more seed‐infected source plants. In two irrigated trials at South Perth, in which healthy and infected seed of cvs Esperance and Green Range was sown, CMV spread was extensive. When the plots were left undefoliated, herbage dry wt yields were decreased by 12 – 30% and seed yields by 53 – 64% due to infection. When they were mown, the herbage dry wt losses recorded were 17 – 24%. In three trials at Mt Barker sown with healthy and infected seed, extensive spread of CMV occurred with cv. Green Range but not with cvs Esperance and Karridale. With cv. Green Range, losses of 25 – 28% in herbage dry wt were recorded inside CMV‐affected patches in mown or grazed plots, while losses were up to 13% when herbage was sampled at random. Seed yield losses were 40–42% and 53% in infected mown and undefoliated cv. Green Range plots, respectively. In the mown or grazed plots of cvs Esperance and Karridale, herbage dry wt losses recorded were up to 7% while seed yield losses were 9 – 16% in mown and 9% in undefoliated plots.The mean wt/seed of seed harvested from mown plots of cvs Green Range and Karridale sown with infected seed was 8–12% less than that of seed from mown control plots. CMV was detected in seed harvested from undefoliated cv. Green Range plots and mown plots of cvs Green Range and Karridale sown with infected seed but levels of seed infection with the mown plots were 3–5 times less than in the seed sown.Field trials were done at two sites in 1987 – 90 to examine the persistence of CMV in subterranean clover swards. CMV infection was established in 1987 and the plots were grazed in subsequent years. At Badgingarra, infection gradually decreased with little CMV being recovered by 1990. At Mt Barker, recovery of CMV was relatively poor in 1988 and even poorer in 1989, but there was some resurgence of CMV infection in 1990.

Response of defoliated swards of subterranean clover and yellow serradella to superphosphate applications

The effect of superphosphate applications (0, 25, 50, 75, 100 and 125 kg P/ha to the soil surface) on the dry matter (DM) herbage production of dense swards of subterranean clover (Trifolium subterraneum cv. Junee) and yellow serradella (Ornithopus compressus cv. Tauro) was measured in a field experiment on deep, sandy soil in south-western Australia. The swards were defoliated with a reel mower at weekly intervals from 88 to 158 days after sowing, to a height of 2 cm for the first 9 cuts, 4 cm for the tenth cut and 5 cm for the eleventh cut. Yellow serradella was more productive than subterranean clover. Consequently, for the relationship between yield and the level of phosphorus (P) applied, yellow serradella supported larger maximum yields and required less P than subterranean clover, to produce the same DM herbage yield. Maximum yields of yellow serradella were 12-40% larger. To produce 70% of the maximum yield for yellow serradella at each harvest, yellow serradella required about 50% less P than subterranean clover. However, when yields were expressed as a percentage of the maximum yield measured for each species at each harvest, the relationship between yield and the level of P applied was similar for both species, and they had similar P requirements.

The maximum yield potential of subterranean clover swards achieved from superphosphate applications is influenced by the maturity and the plant density of the swards

In two field experiments, one conducted in 1987 and the other in 1988, the effect of maturity grading (days from emergence of seedlings to appearance of first flowers) of four clover cultivars (Trifolium subterraneum cvv. Northam, Dalkeith, Junee and Karridale) on the relationship between yield (herbage and seed) and the level of superphosphate applied was measured for dense (170 kg seed ha−1), single-strain, undefoliated swards. In another two field experiments, one conducted in 1987 and the other in 1988, the effect of the density of clover plants, produced by sowing 1, 10, 100 and 1,000 kg seed ha−1 ofT. subterraneum cv. Karridale in 1987 and cv. Junee in 1988, on the relationship between yield (herbage and seed) and the level of superphosphate applied was measured for single-strain, undefoliated swards. In all experiments, phosphorus concentration in dried herbage or seed (tissue test for P) were related to plant yields.

Growth responses of cultivars of subterranean clover to temperature, plant density and nitrate supply

Six cultivars of subterranean clover were compared for growth as swards under controlled conditions in six separate experiments. Conditions common to all experiments were: 400 8mol quanta m-2 s-1 and 12 h day: 12 h night. The variable regimens were: 2 000 or 12 000 plants m-2 ('low' and 'high' density), 10 to 20�C; and no mineral N (0 mM) or N supplied as a 7.5 mM NO3- solution. Growth responses to these conditions were tested by sequential harvesting for dry matter and the fitting of linear and quadratic regression models. Special attention was given to testing for differential responses by cultivars to temperature. All cultivars except 'Woogenellup' grew about 20% faster when NO3- was supplied in the nutrient solution than when the N was derived entirely from N2 fixation under the 0mM treatment. Crop growth rate at 20�C was about double that at 10�C at low density, but at high density the rate at 20�C was 90-95% of that at 10�C. Cultivars differed significantly in growth rate but over all conditions, there was a hierarchical order from the highest to the lowest of: 'Clare', 'Woogenellup', 'Yarloop', 'Mt. Barker', 'Daliak' and 'Geraldton'. There was very little evidence of any cultivar by temperature interaction except for 'Woogenellup, which responded to a low temperature of 10�C compared with a 'high' of 20�C better than the other cultivars. The importance of seed size as a factor influencing the early growth of a subterranean clover sward was emphasized where swards established from large seeds of 'Mt. Barker' grew faster than where small seeds were used. 'Clare' was a highly productive cultivar under all growth conditions. It is concluded that the growth rates of swards of subterranean clover established at seedling densities in excess of 5000 plants m-2 are less sensitive to a low temperature of 10�C than are those established at more conventional densities.

Regrowth by Swards of Subterranean Clover after Defoliation. 1. Growth, Non-structural Carbohydrate and Nitrogen Content

Regrowth by Swards of Subterranean Clover after Defoliation. 1. Growth, Non-structural Carbohydrate and Nitrogen Content

Regrowth by Swards of Subterranean Clover after Defoliation. 2. Carbon Exchange in Shoot, Root and Nodule

Regrowth by Swards of Subterranean Clover after Defoliation. 2. Carbon Exchange in Shoot, Root and Nodule

Effects of Temperature on Growth and Nitrogen Fixation by Swards of Subterranean Clover

Effects of Temperature on Growth and Nitrogen Fixation by Swards of Subterranean Clover

Effects of irrigation timing on seedling establishment and productivity of subterranean clover pastures

Pure swards of subterranean clover were irrigated from late summer-autumn 1982 to autumn 1985. The period of irrigation varied, with 3 times of initial irrigation (1 February, 1 March and 1 April) and 2 times of final irrigation (mid October and mid November). Seedling establishment, DM production and herbage quality were measured in grazed swards. In each regenerating year, seedling density increased as the time of initial irrigation was delayed from February (5.2 seedlingddm2) to April (20.5 seedling/ dm2). For any time of initial irrigation, seedling density was increased by up to 100%, in plots which were irrigated until mid November in the previous spring. Dry matter yield was increased by early irrigation, with average yields of 11.1, 10.3 and 9.1 t/ha.year achieved for February, March and April as times of initial irrigation respectively. Continuing irrigation until November increased annual yield by 0.76 to 1.58 t DM/ha. The late spring irrigation treatment gave increased spring growth plus increased growth in the following autumn as a result of increased seedling density. The quality of the feed produced in autumn was lower than that produced in winter (in vitro DMD 68% compared with 75%), but was higher than the published DMD values for paspalum pastures in autumn. Digestibility in spring declined rapidly, from 75% in September to about 50% in early December. The implications of these findings are discussed with respect to the most effective use of irrigation water, the reliability of subterranean clover regeneration and the timing and quality of the feed produced.

Effects of Nitrate and Ammonium on Nitrogenase (C2H2 Reduction) Activity of Swards of Subterranean Clover, Trifolium subterraneum L

Small swards of subterranean clover plants were grown under controlled conditions without mineral nitrogen and allowed to establish an effective nitrogen fixation system. Nutrient solutions containing nitrate from 0 to 16 mM or of ammonium from 0 to 5 mM were then applied and changes in nitrogenase activity (NA) estimated by acetylene reduction assay (AR) and the rate of hydrogen evolution (HE) for periods of up to 35 days. In two experiments a split-root system was used to enable mineral nitrogen to be applied to only one-half of a nodulated root system whilst the NA of both halves was monitored. NA by subterranean clover was very sensitive to exogenous mineral nitrogen, concentrations as low as 0.5 mM NO3- suppressing activity significantly, and 3-5 mM stopping it almost completely within 7 days. The degree of inhibition induced by concentrations between 0.5 and 3 mM NO3- was less at a photon irradiance of 1000 compared with 300 �mol quanta s-1 m-2 . Under some conditions NA continued at a reduced but steady rate in the presence of nitrate. NH4+ also markedly depressed NA but a concentration greater than 5 mM was needed to effect the same response. After NO3- was applied to an active symbiosis, nitrate reductase activity increased as NA decreased. Our results do not support the hypothesis of a direct effect of NO3- on nitrogenase due to the accumulation of toxic NO2-. Although our results allow that assimilate might be diverted from the nodules after the application of NO3- thus reducing N2-fixation, an alternative hypothesis is proposed: that nitrogenase and nitrate reductase work in a complementary manner in supplying reduced nitrogen to whole plants, and NO3- depresses N2-fixation through a regulatory system involving the level of soluble nitrogen in the plant. We conclude that nitrogen fixation by subterranean clover in the field may be depressed below its potential due to the presence of soil mineral nitrogen.

Comparison of the growth rates of dinitrogen fixing subterranean clover swards with those assimilating nitrate ions

Subterranean clover plants were grown as swards (about 2000 plants/m2) under controlled conditions with N provided either by N2-fixation (NO 3 − withheld) or by assimilation of NO 3 − (NO 3 − supplied). Crop growth rates were measured by dry matter sampling over periods of up to 70 days at PPFD values of 400–1000 μmole quanta/m2/s. When NO 3 − was supplied from sowing the swards grew more rapidly than when the swards were not supplied with NO 3 − and plants had to establish an N2-fixing apparatus. When inter-plant competition was reduced within the sward, a difference in growth rate in favour of NO 3 − -fed plants continued for at least 50 days. When however, a closed canopy was allowed to form, the NO 3 − -fed swards had more dry weight than the N2-fed swards at the time of canopy closure but thereafter the two swards grew at similar rates at light flux densities of above about 800 μmole quanta/m2/s. At light flux densities of about 400 μmole quanta/m2/s N2-fed swards had a growth rate 70–80% of that of NO 3 − -fed plants. NO 3 − -fed plants had a higher organic N content than did N2-fed plants under all conditions.

Effects of temperature on germination, emergence and early seedling growth of swards of Mt Barker subterranean clover plants grown with and without nitrate

Effects of constant temperatures of 10, 15, 20, 25 and 30�C on the germination, emergence and early vegetative growth of Trifolium subterraneum L. cv. Mt Barker grown as swards were examined in temperature-controlled glasshouses and in a growth cabinet. Seedlings were established at a density of about 2000 plants m-2 and grown for up to 70 days. Plants were either inoculated and grown without mineral nitrogen (-N), or supplied with 7.5 mM NO-3 (+ N). Percentage germination and emergence were hardly affected by temperatures of 10-20�C, but at 25�C were reduced to 50%, and at 30�C to about 10%. The rates of germination and emergence were slowest at 10�C, but showed little change with temperature over the range 15-30�C. Time to closed canopy (leaf area index 3) and time to a dry weight of 133 g m-2 were shorter where plants were supplied with NO; than where mineral nitrogen was withheld and a symbiotic system established. Rates of N2-fixation, as measured by acetylene reduction assay, were not markedly affected by temperature over the range 10-25�C. Relative efficiency ranged from about 0.55 at 10, 15, and 20�C to about 0.66 at 25�C. At 30�C nodulation still occurred, but nitrogenase activity was very slight. It is concluded that, where swards of subterranean clover are grown in the absence of any mineral N, a period of N-starvation limits growth during the time taken for symbiotic N2-fixation to become established. Such retardation of growth is small at about 20�C, but becomes more marked at lower and higher temperatures. The establishment of subterranean clover swards in soils of low N status are likely to be retarded following an early (March) or a late (July) start in the growing season. In such cases a 'starter' application of mineral nitrogen may promote the early growth of the legume.